Surface acoustic wave device using a leaky surface acoustic wave with an optimized cut angle of a piezoelectric substrate

ABSTRACT

A surface acoustic wave device includes a piezoelectric substrate of a single crystal LiTaO 3  and an electrode pattern provided on the piezoelectric substrate. The electrode pattern contains Al as a primary component and has a thickness in a range of 0.03-0.15 times a wavelength of a surface acoustic wave excited on the piezoelectric substrate. The piezoelectric substrate has an orientation rotated about an X-axis thereof from a Y-axis thereof toward a Z-axis thereof, with a rotational angle of 39-46°.

BACKGROUND OF THE INVENTION

The present invention generally relates to surface acoustic wave (SAW)devices and more particularly to a SAW device having an improvedpass-band characteristic particularly in a super high frequency bandincluding GHz band.

Surface acoustic wave (SAW) devices are used extensively in highfrequency circuits of compact radio telecommunication apparatusesincluding those for portable use, to form filters and resonators. SuchSAW devices are generally formed on a single crystal or polycrystallinepiezoelectric substrate. Among others, a single crystal substrate ofLiNbO₃ designated as 64° Y-X cut LiNbO₃ (K. Yamanouchi and K. Shibayama,J. Appl. Phys. vol.43, no.3, March 1972, pp.856) and a single crystalsubstrate of LiTaO₃ designated as 36° Y-X cut LiTaO₃ are usedextensively. A 64° Y-X cut LiNbO₃ substrate is a 64° rotated Y-cut plateof a single crystal LiNbO₃ in which the direction of propagation of thesurface acoustic wave is set in the X-direction. On the other hand, a36° Y-X cut LiTaO₃ substrate is a 36° rotated Y-cut plate of a singlecrystal LiTaO₃ in which the direction of propagation of the surfaceacoustic wave is set in the X-direction.

However, these optimized cut angles, used conventionally in thepiezoelectric substrates of LiNbO₃ or LiTaO₃, provide an optimum resultonly when the effect of additional mass, caused by the electrodes on thesubstrate, is ignored. Thus, while the substrates formed with theforegoing, conventional cut angles may provide an optimized result inthe SAW devices for use in a low frequency band lower than severalhundred MHz where the wavelength of the excited surface acoustic wave issufficiently long as compared with the thickness of the electrodes, thesubstrate may be inappropriate for GHz applications as is required inrecent portable telephone systems, due to the thickness of theelectrodes which can no longer be ignored in view of the reducedwavelength of the surface acoustic waves excited therein. In such a highfrequency band, the effect of the mass of the electrode is conspicuous.

It is possible, in a SAW device for use in such a super high frequencyband, to expand the passband of a SAW filter or to decrease acapacitance ratio r of a SAW resonator, when the thickness of theelectrode on the piezoelectric substrate is increased. By doing so, theapparent electromechanical coupling coefficients are increased. However,the SAW device of such a construction raises a problem of increased bulkwave emission from the electrodes, resulting in an increased propagationloss of the surface acoustic wave. The bulk waves emitted from theelectrode as such are called SSBW (surface skimming bulk wave) and thesurface acoustic wave that accompanies a SSBW is called LSAW (leakysurface acoustic wave). As to the propagation loss of the LSAW in a SAWfilter that uses a thick electrode film provided on a 36° Y-X cut LiTaO₃substrate or on a 64° Y-X cut LiNbO₃ substrate, reference should be madeto Plessky et al. (V. S. Plessky and C. S. Hartmann, Proc. 1993 IEEEUltrasonics Symp., pp.1239-1242) and Edmonson et al. (P. J. Edmonson andC. K. Campbell, Proc. 1994 IEEE Ultrasonics Symp., pp.75-79).

In the conventional SAW filters designed for using a LSAW andconstructed on a 36° Y-X cut LiTaO₃ substrate or on a 64° Y-X cut LiNbO₃substrate, it is further noted that the sound velocity of the surfaceacoustic wave is close to the sound velocity of the bulk wave when thethickness of the electrode is small. In such a case, there appears aspurious peak in the vicinity of the passband of the SAW filter due tothe bulk wave emission from the electrode. See Ueda et al. (M. Ueda etal., Proc. 1994 IEEE Ultrasonics Symp. pp.143-146).

FIG. 1 shows the spurious peaks A and B reported by Ueda et al.(op.cit), wherein the spurious peaks A and B are formed in the vicinity ofthe passband of the SAW filter as a result of the bulk wave emission asnoted above. The result of FIG. 1 is obtained for a SAW filter that isformed on a 36° Y-X cut LiTaO₃ substrate and carries thereon aninterdigital electrode of an Al--Cu alloy with a thickness of 0.49 μm.It should be noted that the thickness of the electrode corresponds to 3%of the wavelength of the surface acoustic wave excited in the SAWdevice.

Referring to FIG. 1, it will be noted that the spurious peak B islocated outside the passband formed in the vicinity of 330 MHz, whilethe spurious peak A is formed within the passband and forms anundesirable ripple therein.

As the sound velocity of a SSBW does not change with the thickness ofthe electrode contrary to the sound velocity of a LSAW that changessound velocity depending upon additional mass and hence the thickness ofthe electrode provided on the substrate of a SAW device, the soundvelocity of the LSAW decreases relatively to the sound velocity of theSSBW when the SAW device is operated in a high frequency band such as aGHz band, resulting in a shift of the passband of the SAW filterrelative to the spurious peak B. Thereby, a desirable flat passbandcharacteristic would be obtained for the SAW filter.

However, such an increase of the electrode thickness with respect to thewavelength of the surface acoustic wave leads to the problem ofincreased loss of the LSAW due to the emission of the SSBW as alreadyexplained. Further, such an increase of the electrode thickness resultsin a deterioration of the shape factor of the SAW filter. As will beexplained later, the shape factor of a SAW filter represents thesteepness as well as the width of the passband characteristics of thefilter. More specifically, the filter characteristic becomes broad andundefined when the shape factor of the SAW filter is poor.

Further, in a SAW filter for use in a super high frequency bandincluding GHZ band, it is necessary to secure a certain thickness forthe electrode so as to reduce the resistance of the interdigitalelectrodes. Such a requirement of increased thickness of the electrodeis contradictory with the requirement of reduced loss and improved shapefactor of the SAW device.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea novel and useful SAW device wherein the foregoing problems areeliminated.

Another and more specific object of the present invention is to providea SAW device constructed on a piezoelectric substrate that is cut withan angle optimized with respect to a thickness of an electrode providedthereon.

Another object of the present invention is to provide a SAW devicehaving a pass-band that avoids a spurious peak caused by a bulk waveemission.

Another object of the present invention is to provide a surface acousticwave device, comprising:

a piezoelectric substrate of a single crystal LiTaO₃ ; and

an electrode pattern provided on a surface of said piezoelectricsubstrate and containing Al as a primary component;

said electrode pattern forming a resonator having a thickness in a rangeof about 0.03-0.15 times a wavelength of a surface acoustic wave excitedon said surface of said piezoelectric substrate;

said piezoelectric substrate having an orientation rotated about anX-axis thereof, from a Y-axis thereof, toward a Z-axis thereof, with arotational angle in a range larger than 39° but smaller than about 46°.

Another object of the present invention is to provide a surface acousticwave filter, comprising:

a piezoelectric substrate of a single crystal LiTaO₃ ; and

an electrode pattern provided on a surface of said piezoelectricsubstrate and containing Al as a primary component, said electrodepattern including an interdigital electrode;

said electrode pattern forming a resonator having a thickness in a rangeof about 0.03-0.15 times a wavelength of a surface acoustic wave excitedon said surface of said piezoelectric substrate;

said piezoelectric substrate having an orientation rotated about anX-axis thereof, from a Y-axis thereof, toward a Z-axis thereof, with arotational angle in a range larger than 39° but smaller than about 46°.

Another object of the present invention is to provide a surface acousticwave device, comprising:

a piezoelectric substrate of a single crystal LiTaO₃ ; and

an electrode pattern provided on a surface of said piezoelectricsubstrate and containing Au as a primary component;

said electrode pattern forming a resonator having a thickness in a rangeof 0.004-0.021 times a wavelength of a surface acoustic wave excited onsaid surface of said piezoelectric substrate;

said piezoelectric substrate having an orientation rotated about anX-axis thereof, from a Y-axis thereof, toward a Z-axis thereof, with arotational angle larger than 39° but smaller than about 46°.

Another object of the present invention is to provide a surface acousticwave device, comprising:

a piezoelectric substrate of a single crystal LiTaO₃ ; and

an electrode pattern provided on a surface of said piezoelectricsubstrate and containing Cu as a primary component;

said electrode pattern forming a resonator having a thickness in a rangeof 0.009-0.045 times a wavelength of a surface acoustic wave excited onsaid surface of said piezoelectric substrate;

said piezoelectric substrate having an orientation rotated about anX-axis thereof, from a Y-axis thereof, toward a Z-axis thereof, with arotational angle larger than 39° but smaller than about 46°.

According to the present invention, the angle of cut of the LiTaO₃substrate is optimized with respect to the mass of the electrodeprovided on the surface of the substrate for minimizing the loss.Thereby, one obtains various SAW devices having a broad pass-band andimproved shape factor, including a SAW filter, a SAW resonator and a SAWdelay line.

Another object of the present invention is to provide a surface acousticwave device, comprising:

a piezoelectric substrate of a single crystal LiNbO₃ ; and

an electrode pattern provided on a surface of said piezoelectricsubstrate and containing Al as a primary component;

said electrode pattern having a thickness in a range of about 0.04-0.12times a wavelength of a surface acoustic wave excited on said substrate,

said piezoelectric substrate having an orientation rotated about anX-axis thereof, from a Y-axis thereof, toward a Z-axis thereof, with arotational angle in a range larger than 66° but smaller than about 74°.

Another object of the present invention is to provide a surface acousticwave device, comprising:

a piezoelectric substrate of a single crystal LiNbO₃ ; and

an electrode pattern provided on a surface of said piezoelectricsubstrate and containing Au as a primary component;

said electrode pattern having a thickness in a range of 0.005-0.017times a wavelength of a surface acoustic wave excited on said surface ofsaid piezoelectric substrate;

said piezoelectric substrate having an orientation rotated about anX-axis thereof, from a Y-axis thereof, toward a Z-axis thereof, with arotational angle larger than 66° but smaller than about 74°.

Another object of the present invention is to provide a surface acousticwave device, comprising:

a piezoelectric substrate of a single crystal LiNbO₃ ; and

an electrode pattern provided on a surface of said piezoelectricsubstrate and containing Cu as a primary component;

said electrode pattern having a thickness in a range of 0.012-0.036times a wavelength of a surface acoustic wave excited on said surface ofsaid piezoelectric substrate;

said piezoelectric substrate having an orientation rotated about anX-axis thereof, from a Y-axis thereof, toward a Z-axis thereof, with arotational angle larger than 66 but smaller than about 74°.

According to the present invention, the angle of cut of the LiNbO₃substrate is optimized with respect to the mass of the electrodeprovided on the surface of the substrate for minimizing the loss.Thereby, one obtains various SAW devices having a broad pass-band andimproved shape factor, including a SAW filter, a SAW resonator and a SAWdelay line.

Other objects and further features of the present invention will becomeapparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the passband characteristics of a typicalconventional SAW filter;

FIG. 2 is a diagram explaining the angle of cut of a piezoelectriccrystal;

FIG. 3 is a diagram showing a propagation loss of a SAW device as afunction of the cut angle of a LiTaO₃ substrate for a case in which auniform electrode is provided on the substrate with various thicknesses;

FIG. 4 is a diagram showing a propagation loss of a SAW device as afunction of the cut angle of the LiTaO₃ substrate for a case in which agrating electrode is provided on the substrate with various thicknesses;

FIG. 5 is a diagram showing the relationship between a center frequencyand the temperature for a SAW device constructed on a LiTaO₃ substratewith various cut angles;

FIG. 6 is a diagram showing a temperature dependence of a minimuminsertion loss of a SAW device constructed on a LiTaO₃ substrate havingvarious cut angles;

FIG. 7 is a diagram showing a propagation loss of a SAW device as afunction of the cut angle of a LiNbO₃ substrate for a case in which auniform electrode is provided on the substrate with various thicknesses;

FIGS. 8A and 8B are diagrams showing the construction of a SAW filteraccording to a first embodiment of the present invention respectively ina plan view and in a circuit diagram;

FIG. 9 is a diagram showing the relationship between a minimum insertionloss of a SAW filter and the cut angle of a LiTaO₃ substrate used forthe SAW filter;

FIGS. 10A and 10B are diagrams respectively showing the definition of ashape factor and the relationship between the shape factor and the cutangle of the LiTaO₃ substrate;

FIG. 11 is a diagram explaining the pass-band characteristics of the SAWfilter shown in FIGS.8A and 8B;

FIG. 12 is a diagram showing the relationship between the cut angle ofthe LiTaO₃ substrate and an electromechanical coupling coefficient forthe SAW device shown in FIGS. 8A and 8B;

FIG. 13 is a diagram showing the relationship between the propagationloss and the electrode thickness for various cut angles of the LiTaO₃substrate in the SAW device of FIGS. 8A and 8B;

FIG. 14 is a diagram showing the relationship between the propagationloss and the electrode thickness for various cut angles of the LiNbO₃substrate in the SAW device of FIGS. 8A and 8B;

FIGS. 15A and 15B are diagrams showing a modification of the SAW filterof the first embodiment;

FIG. 16 is an equivalent circuit diagram of a SAW filter according to afurther modification of the first embodiment of the present invention;

FIG. 17 is a diagram showing the construction of a single port SAWresonator according to a second embodiment of the present invention in aplan view; and

FIG. 18 is a diagram showing the construction of a dual port SAWresonator according to a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, the principle of the present invention will be described withreference to FIGS. 2 and 3, wherein FIG. 2 is a diagram that explainsthe cut angle of a piezoelectric substrate.

Referring to FIG. 2 showing a so-called Θ-rotated Y-X cut of a singlecrystal substrate of LiTaO₃ or LiNbO₃, it will be noted that thepiezoelectric substrate is sliced from an ingot of the single crystalLiTaO₃ or LiNbO₃ having crystal axes X, Y and Z, in a state such thatthe substrate is rotated about the X-axis with a rotational angle Θ fromthe Y-axis toward the Z-axis. Thus, the rotational angle Θ is alsocalled the cut angle of the substrate.

FIG. 3 shows the insertion loss of a SAW resonator formed on a Θ-rotatedY-X cut LiTaO₃ substrate, for various cut angles Θ of the substrate.

As explained previously, a 36° Y-X cut substrate is commonly used whenLiTaO₃ is used for the piezoelectric substrate, wherein the particularcut angle of 36° has been used in view of minimization of propagationloss for the surface acoustic wave of relatively long wavelength. SeeNakamura K., et al, Shingaku Gihou US77-42, 1977, pp.31-36 (inJapanese). In the case of a SAW device constructed on a LiNbO₃substrate, a cut angle of 64° has been used commonly for the substrate.

Referring to FIG. 3, the solid circles represent the result ofcalculation of the propagation loss of a LSAW for a SAW device in whicha hypothetical electrode of zero thickness is formed uniformly over thesurface of a 36° Y-X cut substrate of LiTaO₃. The curve represented bythe solid circles clearly indicates the minimum propagation loss at theangle Θ of 36°. In the calculation of FIG. 3, crystal constants reportedby Kovacs were used (Kovacs, G., et al., Proc. 1990 IEEE UltrasonicsSymp. pp.435-438).

In the operation of the SAW device in a super high frequency band suchas a GHz band, however, the thickness of the electrode cannot be ignoredcompared with the wavelength of the surface acoustic wave that isexcited in the SAW device, as already explained. Thus, in the operationof the SAW device in such a super high frequency band, the effect of theadded mass of the electrode is conspicuous. The inventors of the presentinvention have discovered that, because of the effect of such an addedmass of the electrode, the curve representing the propagation loss inFIG. 3 shifts in the direction indicated by an arrow in FIG. 3, from thecurve represented by the solid circles to a curve represented by opencircles. Associated with the shift of the transmission loss as such, theoptimum angle Θ that provides the minimum transmission loss increasesfrom 36° to 38° or more. In FIG. 3, the result represented by the opencircles is for the case in which the electrode is provided on thepiezoelectric substrate with a thickness of about 10% the wavelength ofthe excited surface acoustic wave.

FIG. 4 shows the propagation loss for the case in which a gratingelectrode of Al is provided on a LiTaO₃ substrate as a function of thecut angle Θ of the substrate. In FIG. 4, the broken line represents theresult in which the thickness of the Al electrode is zero, while thecontinuous line indicates the result in which the electrode has anormalized thickness of 10% with respect to the wavelength of theexcited surface wave. Obviously, the cut angle that provides the minimumof the propagation loss is shifted to the side of higher cut angle whena grating electrode of finite thickness is provided on the substrate.

Thus, the results of FIGS. 3 and 4 clearly indicate that one can realizea SAW device having a high Q-factor and hence low attenuation of surfaceacoustic waves in a GHz band by setting the cut angle Θ higher than theconventionally used cut angle of 36° when using a single crystal LiTaO₃for the piezoelectric substrate. Further, associated with the added masseffect of the electrode in such a super-high frequency, the passband ofthe SAW filter shown in FIG. 1 shifts in the direction of the lowerfrequency side with respect to the spurious peaks A and B, resulting ina SAW device substantially free from ripples in the pass-band. As notedalready, the spurious peaks A and B are caused by the bulk wave emissionand are not affected by the added mass of the electrode.

Further, the inventors have discovered that the shape factor of thepass-band changes also with the cut angle Θ. More specifically, the SAWfilter constructed on a LiTaO₃ substrate with the cut angle Θ largerthan the conventionally used cut angle, provides not only an improvedpassband characteristic but also an improved shape factor in theoperation of near GHz bands.

FIGS. 5 and 6 show respectively the temperature dependence of the centerfrequency and the minimum insertion loss for a SAW filter constructed ona LiTaO₃ substrate. In the experiments of FIGS. 5 and 6, a SAW filter tobe described later with reference to FIGS. 8A and 8B was used, in whichthe electrode was formed on various LiTaO₃ substrates having various cutangles (36° Y, 40° Y, 42° Y, 44° Y), with a normalized thickness ofabout 10% the wavelength of the surface acoustic wave excited on thesubstrate.

As will be noted clearly from FIG. 5, the SAW filter shows substantiallythe same temperature dependence of the center frequency, irrespective ofthe cut angle of the substrate. The observed scattering of the centralfrequency is attributed to the variation of the sound velocity in thesubstrate and the preparation process of the device.

Further, FIG. 6 indicates that, at least in the normal temperature rangeof -35° C.-85° C., one can reduce the minimum insertion loss withrespect to the device constructed on a 36° Y-X LiTaO₃ substrate, bysetting the cut angle to 40° Y-44° Y. Particularly, it can be seen thatthe magnitude of the variation of the minimum insertion loss is reducedalso by setting the cut angle to the range between 40° Y and 42° Y.

FIG. 7 shows the insertion loss of a SAW resonator constructed on a Y-Xsubstrate of LiNbO₃ for various rotation angles Θ.

Referring to FIG. 7, the curve represented by a broken line indicates acalculated propagation loss of a LSAW for the case in which a uniformelectrode is provided on a 64° Y-X substrate of LiNbO₃, with a zerothickness of the electrode. The result of FIG. 7 indicates that aminimum of propagation loss is achieved by setting the cut angle Θ to64°. It should be noted that the calculation of FIG. 7 is made by usingthe crystal constants reported by Warner, et al, J. Acoustic. Soc.Amer., 42, 1967, pp.1223-1231.

In the case of the operation in a shorter wavelength region such as GHzband, the effect of the thickness of the electrode is no longernegligible in view of the increased relative thickness of the electrodewith respect to the wavelength of the excited surface wave. The inventorof the present invention has discovered that, because of such an addedmass effect of the electrode, the characteristic curve of FIG. 7 shiftsto the higher side of the cut angle Θ as indicated by an arrow in FIG.7. Associated therewith, the cut angle Θ that provides the minimumpropagation loss shifts also in the higher angle side, as indicated inFIG. 7 by a continuous line. In FIG. 7, it should be noted that thecontinuous line represents the case in which the electrode has athickness of about 3% the wavelength of the surface acoustic waveexcited on the substrate.

The result of FIG. 7 clearly indicates that one can obtain a high Q SAWdevice that shows a reduced attenuation of the surface acoustic wave ina GHz band, by setting the cut angle Θ of a single crystal LiNbO₃ to behigher than 64°.

Hereinafter, the present invention will be described in detail withreference to preferred embodiments.

FIGS. 8A and 8B show a ladder type SAW filter according to a firstembodiment of the present invention, wherein FIG. 8A shows the layout ofthe SAW filter in a plan view while FIG. 8B shows an equivalent circuitdiagram of the device of FIG. 8A.

Referring to FIG. 8A, the SAW filter is formed on a Θ-rotated Y-X cutsubstrate of a LiTaO₃ single crystal or a LiNbo₃ single crystal andcarries thereon a first interdigital electrode R₁ having an input-sideelectrode connected to an input terminal IN provided on the substrate, asecond interdigital electrode R₁ ' having an input-side electrodeconnected to an output-side electrode of the interdigital electrode R₁and an output-side electrode connected to an output terminal OUTprovided on the substrate, a third interdigital electrode R₂ having aninput-side electrode connected to the foregoing input-side electrode ofthe interdigital electrode R₁ ' and a grounded output-side electrode, afourth interdigital electrode R₂ ' having an input-side electrodeconnected to the foregoing output-side electrode of the interdigitalelectrode R₁ and a grounded output-side electrode, and a fifthinterdigital electrode R₂ " having an input-side electrode connected tothe output-side electrode of the interdigital electrode R₁ ' and agrounded output-side electrode.

In each of the interdigital electrodes R₁, R₁ ', R₂, R₂ ', and R₂ ", itwill be noted that the interdigital electrode includes an input-sideelectrode i and an output-side electrode o as indicated in FIG. 8A. Theinput-side electrode i includes first group of electrode fingersextending parallel with each other in a first direction so as to crossthe path of the surface acoustic wave propagating on the surface of thesubstrate in the X-axis direction. Similarly, the output-side electrodeo includes second group of electrode fingers extending parallel witheach other in a second, opposite direction, wherein the first group ofelectrode fingers and the second group of electrode fingers are disposedalternately on the substrate surface in the propagating direction of thesurface acoustic wave. Further, each of the interdigital electrodes R₁,R₁ ', R₂ and R₂ ' is accompanied by a pair of reflectors R_(e) disposedat both sides thereof in the X-axis direction. Each of the reflectorsR_(e) has a construction in which a plurality of mutually parallelelectrode fingers are connected with each other at both ends of theelectrode fingers. In the present embodiment, it should be noted thatthe interdigital electrodes R₁, R₁ ', R₂ and R₂ ' are formed of analuminum alloy containing Al and 1 wt % of Cu, and has a thickness ofabout 0.4 μm corresponding to about 10% of the passband wavelength ofthe SAW filter.

FIG. 8B shows the equivalent circuit diagram of the filter of FIG. 8A.

Referring to FIG. 8B, the interdigital electrodes R₁ and R₁ ' areconnected in series, while the interdigital electrodes R₂, R₂ ' and R₂ "are connected parallel with each other at both sides of the interdigitalelectrode R₁ ' or R₁.

FIG. 9 shows the minimum insertion loss obtained experimentally for theSAW filter of FIGS. 8A and 8B for various cut angles Θ of the LiTaO₃substrate 11. The minimum insertion loss includes contributions of boththe propagation loss of the surface acoustic wave and the filtermatching loss, while the filter matching loss is not affected by the cutangle Θ.

Referring to FIG. 9, the minimum insertion loss decreases withincreasing cut angle of the substrate and reaches a minimum in thevicinity of 42°. When the cut angle Θ exceeds 42°, the minimum insertionloss starts to increase again. Thus, it will be noted that one cansuppress the minimum insertion loss of the SAW filter within 1.6 dB bysetting the cut angle of the LiTaO₃ in the range between 38° and 46°.

The inventors of the present invention have discovered that the cutangle Θ of a LiTaO₃ single crystal substrate also affects the shapefactor of the SAW filter.

FIG. 10A shows the definition of the shape factor.

Referring to FIG. 10A, a shape factor is defined, in terms of bandwidthsBW₁ and BW₂, as BW₁ /BW₂, in which the bandwidth BW₁, corresponds to abandwidth that provides an attenuation of 1.5 dB, while the bandwidth B₂corresponds to a bandwidth that provides an attenuation of 20 dB. Withincreasing shape factor, the filter characteristics become broad,resulting in a deterioration of the selectivity and narrowing of thepassband. Thus, it is desirable for a SAW filter to have a shape factoras close to 1 as possible.

FIG. 10B shows the shape factor obtained experimentally for the SAWfilter of FIGS. 8A and 8B as a function of the cut angle Θ of thepiezoelectric substrate 11.

As will be noted from FIG. 10B, the shape factor approaches 1 withincreasing cut angle Θ and reaches a minimum of 1.47 at the cut angle Θof 42°. On the other hand, when the cut angle exceeds the foregoingangle of 42°, the shape factor starts to increase again, leading to adeterioration of the filter selectivity. In the SAW filter of thepresent invention, therefore, it is desirable to have the minimuminsertion loss of 1.6 dB or less and the shape factor of 1.55 or less.In view of the relationship of FIG. 10B, therefore, one obtains anoptimum cut angle Θ of 40-46°, particularly in the range of 40-44°. Bysetting the cut angle Θ to be 42° in particular, the minimum insertionloss is minimized and simultaneously the shape factor.

FIG. 11 shows a passband characteristic obtained experimentally for theSAW filter of FIGS. 8A and 8B. Referring to FIG. 11, the continuous linerepresents the case in which a 42° Y-X cut LiTaO₃ is used for thesubstrate 11, while the one-dotted broken line represents the case inwhich a conventional 360 Y-X cut LiTaO₃ is used for the substrate 11.

Referring to FIG. 10, it will be noted that both SAW filters have acentral frequency at 880 MHz and are characterized by a flat passband ofabout 40 MHz in width, defined by a sharp attenuation slope outside thepass-band, wherein the SAW filter that uses the 42° Y-X cut plate ofLiTaO₃ for the substrate 11 provides an improved shape factoraccompanied with a steeper attenuation slope outside the passband ascompared with the conventional SAW filter that uses the conventional 36°Y-X cut plate of LiTaO₃ for the substrate 11. Further, as indicated inFIG. 11, the spurious peaks A and B caused by SSBW are now locatedoutside the pass-band of the SAW filter formed on the 42° Y-X LiTaO₃substrate.

FIG. 12 shows the result of calculation of the electromechanicalcoupling coefficients k² of a Θ-rotated Y-X cut LiTaO₃ substratecarrying thereon an electrode having a thickness of about 7% of thesurface acoustic wave that is excited thereon, for various cut angles Θ.The calculation was made by using the crystal constants reported byKovacs op. cit.

Referring to FIG. 12, it will be noted that the electromechanicalcoupling coefficients k² show a tendency to decrease with increasing cutangle Θ. As is well known, the electromechanical coupling coefficientsk² represent the ratio of the energy accumulated in a piezoelectriccrystal due to the piezoelectric effect. When the value of k² is toosmall, various problems such as reduced pass band, appearance of ripplein the passband, and the like appear. From the relationship of FIG. 12,therefore, it is desirable to set the cut angle Θ not to exceed 46°.

FIG. 13 shows the result of calculation of the propagation loss on a SAWfilter of FIGS. 8A and 8B for various cut angles Θ and for variousthicknesses of the interdigital electrodes. In the calculation of FIG.9, too, the crystal constants of Kovacs were used.

As will be noted from FIG. 13, the loss increases exponentially with thethickness of the electrode when the cut angle Θ is set below 38°. Whenthe cut angle Θ exceeds 40°, on the other hand, the loss starts todecrease with increasing thickness of the electrode and a minimumappears in the characteristic curve. Particularly, such a minimumappears for the thickness of the electrode set larger than about 3% ofthe surface acoustic wavelength. In other words, FIG. 13 indicates thatit is desirable to form the electrode such that the thickness,normalized by the wavelength of the surface acoustic wave, is equal toor larger than 3%. When the thickness of the electrode is excessive,problems such as difficulty in patterning the electrode or substantialchange of the sound velocity with minute change of the electrodethickness appear. Thus, it is preferred to form the electrode such thatthe electrode thickness does not exceed 15% of the wavelength of thesurface acoustic wave.

FIG. 13 also indicates that the propagation loss increases steeply atany cut angles when the thickness of the electrode of Al-alloy exceeds15% of the wavelength of the surface acoustic wave. This indicates thatemission of bulk waves becomes predominant under such conditions. Thus,in view of the foregoing, it is desirable to set the electrode thicknessto fall in the range of 7-15% of the surface acoustic wavelength whenthe cut angle Θ is in the range between 40° and 46° and moreparticularly in the range of 5-10% of the surface acoustic wavelengthwhen the cut angle Θ is in the range between 40°-44°.

FIG. 14 shows the result of the calculation of the propagation loss fora SAW filter having the construction of FIGS. 8A and 8B except that aY-X LiNbO₃ single crystal is used for the substrate 11 in place of theY-X LiTaO₃ while changing the thickness of the electrode on thesubstrate 11. In the calculation of FIG. 14, the crystal constantsreported by Warner et al., op cit. was used.

Referring to FIG. 14, it will be noted that the propagation loss firstdecreases to a minimum and then starts to increase exponentially with anincrease of the electrode thickness, wherein the minimum of thepropagation loss appears at the conventional optimum cut angle of 64° orless only when the electrode thickness is less than 3.5% the wavelengthof the excited surface acoustic wave. On the other hand, when thethickness of the electrode is larger than about 4% the wavelength of theexcited surface acoustic wave, the minimum of the propagation lossappears at a cut angle exceeding 66°. In other words, under theoperational condition of the SAW device in which the thickness of theelectrode cannot be ignored with respect to the wavelength of theexcited surface acoustic wave, it is desirable to set the cut angle ofthe LiNbO₃ substrate to be larger than about 66°.

On the other hand, when the thickness of the electrode is excessive, thesound velocity in the substrate may be affected by the thickness of theelectrode. Further, such a thick electrode may cause a difficulty inpatterning the electrode. Because of this reason, it is desirable to setthe thickness of the electrode not to exceed 12% of the wavelength ofthe excited surface acoustic wave. Associated with this, it ispreferable to set the cut angle of the LiNbO₃ substrate to fall in arange between 66° and 74°.

In the foregoing description, it was assumed that the electrode isformed of an aluminum alloy containing Al and 1 wt % of Cu (Al-1% Cu).When other composition and hence a different mass is used for theelectrode, the thickness of the electrode should be changed accordingly.For example, when Au is used for the electrode on a LiTaO₃ substrate,the electrode thickness is preferably chosen to fall in the rangebetween 0.4 and 2.1% of the wavelength. Further, when Cu is used for theelectrode on a LiTaO₃ substrate, the electrode thickness is preferablychosen to fall in the range between 0.9 and 4.5% of the wavelength ofthe surface acoustic wave.

When a Y-X LiNbO₃ single crystal is used for the substrate of the SAWdevice, on the other hand, it is preferable to set the thickness of anAu electrode to fall in the range between 0.5 and 1.7%. When forming aCu electrode on the LiNbO₃ substrate, it is preferable to set thethickness of the electrode in the range between 1.2 and 3.6% of thewavelength of the surface acoustic wave.

FIG. 15A shows a modification of the SAW filter of FIG. 8A while FIG.15B shows the equivalent circuit diagram of the SAW filter of FIG. 15A.

Referring to FIG. 15A, the SAW filter is constructed on a Y-X singlecrystal substrate of LiTaO₃ or LiNbO₃, similarly to the previousembodiment, wherein the substrate carries thereon a first interdigitalelectrode R₁ having an input-side electrode connected to an inputterminal IN provided on the substrate, a second interdigital electrodeR₁ ' having an input-side electrode connected to an output-sideelectrode of the interdigital electrode R₁ and an output-side electrodeconnected to an output terminal, a third interdigital electrode R₂ 'having an inputside electrode connected to the foregoing output-sideelectrode of the interdigital electrode R₁ and a grounded output-sideelectrode, and a fourth interdigital electrode R₂ having an input-sideelectrode connected to the output-side electrode of the interdigitalelectrode R₁ " and a grounded output-side electrode.

Referring to FIG. 15B, the interdigital electrodes R₁ and R₁ ' areconnected in series, while the interdigital electrodes R₂ and R₂ ' areconnected parallel with each other at both sides of the interdigitalelectrode R₁ '. It should be noted that each of the interdigitalelectrodes R₁, R₁ ', R₂ and R₂ ' forms a resonator, and the interdigitalelectrode R₁ ' has a capacitance about one-half the capacitance of theinterdigital electrode R₁. On the other hand, the interdigital electrodeR₂ ¹ ' has a capacitance twice as large as the capacitance of theinterdigital electrode R₂.

In the SAW filter of FIGS. 15A and 15B, too, it is possible to minimizethe propagation loss in a GHz band operation in which the added masseffect of the electrode becomes conspicuous, by setting the cut angle ofthe LiTaO₃ substrate to be larger than 38° but smaller than about 46°,preferably larger than about 40° but smaller than about 46°, mostpreferably to about 42°. In the case a Y-X LiNbO₃ is used for thesubstrate, on the other hand, it is preferable to set the cut angle tobe larger than 66° but smaller than about 74°, more preferably to about68°, when the SAW filter is used in the frequency region where the addedmass effect is conspicuous.

It should be noted that the present invention is by no means limited tothe foregoing ladder type SAW filter but is also applicable to SAWfilters of other types, various SAW resonators and SAW delay lines. Forexample, one may modify the electrode pattern of the SAW filter of FIGS.8A and 8B to form a lattice type SAW filter shown in FIG. 16.

FIG. 17 shows the construction of a single port SAW resonator 50according to a second embodiment of the present invention.

Referring to FIG. 17, the SAW resonator 50 is constructed on theΘ-rotated Y-X cut substrate 11 of LiTaO₃ having the cut angle Θ of38-46°, wherein the substrate 11 carries thereon an interdigitalelectrode R having a thickness of about 3-15% of the wavelength of theLSAW excited on the substrate 11. Alternatively, the substrate 11 may beformed of a Y-X cut single crystal of LiNbO₃ having the cut angle Θ of66-74°. In this case, the electrode on the substrate 11 has a thicknessof about 4-12% of the wavelength of the LSAW excited on the substrate11. In the present embodiment also, the LSAW propagates in theX-direction.

It will be noted that the SAW resonator 50 carries a single interdigitalelectrode R and a pair of reflectors R_(e) are disposed at both sides ofthe electrode R in the X-direction. The resonator 50 is thereby drivenby applying a voltage across a first terminal 51 connected to the firstgroup of electrode fingers of the interdigital electrode R and a secondterminal 52 connected to the second group of electrode fingers of thesame interdigital electrode R.

In such a construction, it is possible to provide a high Q resonatorhaving a minimum loss, by optimizing the cut angle Θ of the substrateand the thickness of the electrode similarly to the embodiment of FIGS.8A and 8B.

FIG. 18 shows the construction of a dual port SAW resonator 60 accordingto a third embodiment of the present invention.

Referring to FIG. 18, the SAW resonator 60 is constructed on theΘ-rotated Y-X cut substrate 11 of LiTaO₃ having the cut angle Θ of38-46°, wherein the substrate 11 carries thereon an interdigitalelectrode having a thickness of about 3-15% of the wavelength of theLSAW excited on the substrate 11. Alternatively, the substrate 11 may beformed of a Y-X cut single crystal of LiNbO₃ having the cut angle Θ of66-74°. In this case, the electrode on the substrate 11 has a thicknessof about 4-12% of the wavelength of the LSAW excited on the substrate11. In the present embodiment also, the LSAW propagates in theX-direction.

In FIG. 18 it will be noted that the interdigital electrode of the SAWresonator 60 includes an input electrode R₁ connected to an inputterminal 61 and an output electrode R₂ connected to an output terminal62, wherein the input terminal 61 and the output terminal 62 aredisposed in the propagating direction X on the surface acoustic wave.Further, each of the interdigital electrodes R₁ and R₂ is formed of afirst interdigital electrode and a second interdigital electrode,wherein the first interdigital electrode of the electrode R₁ isconnected to the input terminal 61 while the first interdigitalelectrode of the electrode R₂ is connected to the output terminal 62.Further, the second interdigital electrode of the electrode R₁ and thesecond interdigital electrode of the electrode R₂ are connected witheach other to form a single ground pattern. At both sides of theinterdigital electrode thus formed of the electrode R₁ and the R₂, apair of reflectors R_(e) are disposed as usual.

In the dual port SAW resonator 60, too, it is possible to maximize theQ-factor and minimize the loss by optimizing the cut angle of thesubstrate and the thickness of the interdigital electrode similarly tothe SAW filter of FIGS. 8A and 8B.

Thus, the present invention is applicable to various SAW filters andresonators such as the one having the ladder construction as in the caseof FIGS. 8B and 15B, or the lattice construction as in the case of FIG.16. Further, the present invention is applicable to multiple modefilters.

Further, the present invention is not limited to the SAW filter and SAWresonator described heretofore, but is applicable also to SAW delaylines and SAW waveguides as well.

Further, the present invention is by no means not limited to theembodiments described heretofore, but variations and modifications maybe possible without departing from the scope of the invention.

What is claimed is:
 1. A surface acoustic wave device, comprising:apiezoelectric substrate of a single crystal LiTaO₃ ; and an electrodepattern provided on a surface of said piezoelectric substrate andcontaining Al as a primary component; said electrode pattern forming aresonator having a thickness in a range of about 0.03-0.15 times awavelength of a surface acoustic wave excited on said surface of saidpiezoelectric substrate; said piezoelectric substrate having anorientation rotated about an X-axis thereof, from a Y-axis thereof,toward a Z-axis thereof, with a rotational angle more than 39° and lessthan about 46°.
 2. A surface acoustic wave device as claimed in claim 1,wherein said electrode pattern forms a plurality of resonators, saidpiezoelectric substrate has an orientation in which said rotationalangle is set in the range of about 40-46°.
 3. A surface acoustic wavedevice as claimed in claim 1, wherein said electrode pattern has athickness in the range of about 0.07-0.15 times the wavelength of thesurface acoustic wave excited on said piezoelectric substrate.
 4. Asurface acoustic wave device as claimed in claim 1, wherein saidelectrode pattern has a thickness in the range of about 0.05-0.10 timesthe wavelength of said surface acoustic wave excited on saidpiezoelectric substrate, and wherein said piezoelectric substrate has anorientation in which said rotational angle is set in the range of about40-44°.
 5. A surface acoustic wave device as claimed in claim 1, whereinsaid piezoelectric substrate has an orientation in which said rotationalangle is set to about 42°.
 6. A surface acoustic wave device as claimedin claim 1, wherein said electrode pattern is formed of Al.
 7. A surfaceacoustic wave device as claimed in claim 1, wherein said electrodepattern is formed of an aluminum alloy containing copper.
 8. A surfaceacoustic wave device as claimed in claim 1, wherein said electrodepattern forms a plurality of resonators connected in a ladder type onthe surface of said piezoelectric substrate.
 9. A surface acoustic wavefilter, comprising:a piezoelectric substrate of a single crystal LiTaO₃; and an electrode pattern provided on a surface of said piezoelectricsubstrate and containing Al as a primary component, said electrodepattern including an interdigital electrode; said electrode patternforming a resonator and having a thickness in a range of about 0.03-0.15times a wavelength of a surface acoustic wave excited on said surface ofsaid piezoelectric substrate; said piezoelectric substrate having anorientation rotated about an X-axis thereof, from a Y-axis thereof,toward a Z-axis thereof, with a rotational angle more than 39° and lessthan about 46°.
 10. A surface acoustic wave resonator, comprising:apiezoelectric substrate of a single crystal LiTaO₃ ; and an electrodepattern provided on a surface of said piezoelectric substrate andcontaining Al as a primary component, said electrode pattern includingan interdigital electrode; said electrode pattern having a thickness ina range of about 0.03-0.15 times a wavelength of a surface acoustic waveexcited on said surface of said piezoelectric substrate; saidpiezoelectric substrate having an orientation rotated about an X-axisthereof, from a Y-axis thereof, toward a Z-axis thereof, with arotational angle more than 39° and less than about 46°; saidinterdigital electrode including a first electrode group and a secondelectrode group, said first electrode group including a first group ofelectrode fingers provided on said surface of said piezoelectricsubstrate and connected commonly to a first terminal, said secondelectrode group including a second group of electrode fingers providedon said surface of said piezoelectric substrate and connected commonlyto a second terminal, said first and second group of electrode fingersbeing disposed such that a second group electrode finger is interposedbetween a pair of first group of electrode fingers.
 11. A surfaceacoustic wave device, comprising:a piezoelectric substrate of a singlecrystal LiTaO₃ ; and an electrode pattern provided on a surface of saidpiezoelectric substrate and containing Au as a primary component; saidelectrode pattern forming a resonator and having a thickness in a rangeof 0.004-0.021 times a wavelength of a surface acoustic wave excited onsaid surface of said piezoelectric substrate; said piezoelectricsubstrate having an orientation rotated about an X-axis thereof, from aY-axis thereof, toward a Z-axis thereof, with a rotational angle morethan 39° and less than about 46°.
 12. A surface acoustic wave device,comprising:a piezoelectric substrate of a single crystal LiTaO₃ ; and anelectrode pattern provided on a surface of said piezoelectric substrateand containing Cu as a primary component; said electrode pattern forminga resonator and having a thickness in a range of 0.009-0.045 times awavelength of a surface acoustic wave excited on said surface of saidpiezoelectric substrate; said piezoelectric substrate having anorientation rotated about an X-axis thereof, from a Y-axis thereof,toward a Z-axis thereof, with a rotational angle more than 39° but lessthan about 46°.
 13. A surface acoustic wave device, comprising:apiezoelectric substrate of a single crystal LiNbO₃ ; and an electrodepattern provided on a surface of said piezoelectric substrate andcontaining Au as a primary component; said electrode pattern having athickness in a range of 0.005-0.017 times a wavelength of a surfaceacoustic wave excited on said surface of said piezoelectric substrate;said piezoelectric substrate having an orientation rotated about anX-axis thereof, from a Y-axis thereof, toward a Z-axis thereof, with arotational angle larger than 66° but smaller than about 74°.
 14. Asurface acoustic wave device, comprising:a piezoelectric substrate of asingle crystal LiNbO₃ ; and an electrode pattern provided on a surfaceof said piezoelectric substrate and containing Cu as a primarycomponent; said electrode pattern having a thickness in a range of0.012-0.036 times a wavelength of a surface acoustic wave excited onsaid surface of said piezoelectric substrate; said piezoelectricsubstrate having an orientation rotated about an X-axis thereof, from aY-axis thereof, toward a Z-axis thereof, with a rotational angle largerthan 66° but smaller than about 74°.
 15. A surface acoustic wave device,comprising:a piezoelectric substrate of a single crystal LiNbO₃ ; and anelectrode pattern provided on a surface of said piezoelectric substrateand containing Al as a primary component; said electrode pattern havinga thickness in a range of about 0.04-0.12 times a wavelength of asurface acoustic wave excited on said substrate, said piezoelectricsubstrate having an orientation rotated about an X-axis thereof, from aY-axis thereof, toward a Z-axis thereof, with a rotational angle in arange larger than 66° but smaller than about 74°.
 16. A surface acousticwave device as claimed in claim 15, wherein said electrode pattern formsa resonator on said surface of said piezoelectric substrate.
 17. Asurface acoustic wave device as claimed in claim 15, wherein saidelectrode pattern forms a lattice filter including a plurality ofresonators on said surface of said piezoelectric substrate.
 18. Asurface acoustic wave device as claimed in claim 15, wherein saidelectrode pattern forms a multi-mode filter on said surface of saidpiezoelectric substrate.
 19. A surface acoustic wave device as claimedin claim 15, wherein said electrode pattern includes first through fifthinterdigital electrodes disposed on said surface of said piezoelectricsubstrate along a path of said surface acoustic wave,said firstinterdigital electrode comprising a plurality of a first group ofelectrode fingers connected commonly to an input terminal and aplurality of a second group of electrode fingers intervening betweensaid first group of electrode fingers, each of said first and secondgroup of electrode fingers crossing said path of said surface acousticwave on said surface of said substrate, said second interdigitalelectrode comprising a plurality of a third group of electrode fingersconnected commonly to said second group electrode fingers and aplurality of a fourth group of electrode fingers connected commonly toan output terminal and intervening between said third group of electrodefingers, each of said third and fourth group of electrode fingerscrossing said path of said surface acoustic wave on said surface of saidsubstrate, said third interdigital electrode comprising a plurality of afifth group of electrode fingers connected commonly to said first groupof electrode fingers and a plurality of a sixth group of electrodefingers connected commonly to a ground and intervening between saidfifth group of electrode fingers, each of said fifth and sixth group ofelectrode fingers crossing said path of said surface acoustic wave onsaid surface of said substrate, said fourth interdigital electrodecomprising a plurality of a seventh group of interdigital electrodefingers connected commonly to said second group of electrode fingers anda plurality of an eighth group of electrode fingers connected commonlyto the ground and intervening between said seventh group of interdigitalelectrode fingers, each of said seventh and eighth group of electrodefingers crossing said path of said surface acoustic wave on said surfaceof said substrate, said fifth interdigital electrode comprising aplurality of a ninth group of interdigital electrode fingers connectedcommonly to said fourth group of electrode fingers and a tenth group ofelectrode fingers connected commonly to the ground and interveningbetween said ninth group of electrode fingers, each of said ninth andtenth group of electrode fingers crossing said path of said surfaceacoustic wave on said substrate.
 20. A surface acoustic wave device asclaimed in claim 15, wherein said piezoelectric substrate has anorientation in which said rotational angle is set in the range of aboutof about 68-72°.
 21. A surface acoustic wave device as claimed in claim15 wherein said electrode pattern has a thickness in the range of about0.05-0.10 times the wavelength of said surface acoustic wave excited onsaid piezoelectric substrate.
 22. A surface acoustic wave device asclaimed in claim 15, wherein said electrode pattern is formed of Al. 23.A surface acoustic wave device as claimed in claim 15, wherein saidelectrode pattern is formed of an aluminum alloy containing copper. 24.A surface acoustic wave device as claimed in claim 15, wherein saidelectrode pattern forms a ladder filter including a plurality ofresonators on said surface of said piezoelectric substrate.